116 III. OXIDATION AND METABOLISM 



linoleic acid to V or VI, whereby these latter compounds are changed to VII 

 and VIII, respectively. Thus, linoleic acid is changed to II, which can then 

 be further oxidized by either of the above pathways. It has been suggested 

 that the function of lipoxidase may be to initiate the chain reactions by re- 

 moving the hydrogen atom from the linoleic acid methylene group. 



However, Tappel, Boyer, and Lundberg-^^ noted several differences in the 

 decomposition of linoleate brought about by autoxidation and by catalysis 

 with lipoxidase, respectively. For example, the extinction coefficient at 

 232.5 m/i for the peroxides formed in the presence of lipoxidase was higher 

 than that which resulted from the autoxidation of linoleate. Antioxidants 

 inhibited the oxidation of linoleate as the result of a direct effect upon 

 lipoxidase and because of a preferential oxidation of the antioxidant. To 

 replace the assumption that the oxidation of linoleate proceeds as a chain 

 reaction, Tappel et al.^^^ suggest the formation of a biradical from linoleate 

 and oxygen on the enzyme surface. The biradical may accept electrons 

 from antioxidants, or may react to give the conjugate linoleate peroxide. 



On the other hand, Klian-" noted that the infrared absorption spectra of 

 the peroxide concentrate (reduced) from methyl linoleate autoxidized be- 

 low 0°C. with dispersed oxygen, and the peroxide concentrate (reduced) 

 from linoleic acid oxidized at 0°C. in the presence of lipoxidase with dis- 

 persed oxygen, are identical at the same wave lengths, proving the presence 

 of the two major isomers, cis-trans conjugated and trans-trans conjugated. 

 Thus, although lipoxidase catalysis may have proceeded through the con- 

 trolled mechanisms proposed by Tappel et al.,-^^ it has also stabilized the 

 free radicals at the 9- and 13-positions of the linoleate system, producing the 

 conjugated isomers.-" The free radicals initiated at the 11-position are 

 able to change over to the more stable forms. 



af . Coupled Reactions: There is considerable evidence that a coupled 

 oxidation occurs in a number of easily oxidized compounds and linoleic acid 

 under the action of lipoxidase. Substances which have been found to exert 

 this effect include various carotenoids,^^^-^^^ chlorophyll,'^^ hemin,^^^ 

 ascorbic acid'^^ (although slowly) and certain dyes.'^^ 



Holman^^^ reported that, in the coupled oxidation of carotene and linoleic 

 acid with pure lipoxidase, the amount of carotene destroyed and the amount 

 of diene conjugated are proportional to the enzyme concentration and to the 



"« A. L. Tappel, P. D. Boyer, and W. O. Lundberg, /. Biol. Chem., 199, 267-281 

 (1952). 



"' N. A. Khan, Arch. Biochem. Biophys., 44, 247-249 (1953). 



238 M. W. Kies, Federation Proc, 5, 141 (1946). 



"9 R. T. Holman, Federation Proc, 7, 160-161 (1948). 



